Institut des
NanoSciences de Paris
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Coherent manipulation of qubits in semiconductor quantum dots. Spin-photon interfaces

P. Atkinson, B. Eble, R. Hostein, F. Dubin, V. Voliotis

Semiconductor quantum dots can be considered in first approximation as two-level systems and define in that way qubits. A photo-excited electron-hole pair or the spin of a charge carrier (electron or hole) can define two different qubits with different coherence properties. In order to realize logic operations, the qubit must be initialized in a given state and manipulated (rotations on the Bloch sphere) rapidly during its coherence time. This can be achieved using optical pulses in coherent control schemes that we have developed the past years in our group. Resonant luminescence experiments allowed demonstrating Rabi oscillations of the neutral electron-hole pair in a dot, showing that the qubit can be prepared in any desired superposition, controlled only by the pump parameters (polarization, power). Two laser pulses with a controlled delay and relative phase can then achieve the coherent control of the qubit. All the dephasing processes occurring in solid matter affect the coherence properties of the system and limit these manipulations. This is an important issue to address in order to optimize the fidelity of the operations [1-2].

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Caption : schematic representation of a two-level system resonantly excited by laser pulses. The Rabi oscillation between the ground state (empty QD-state 0) and the excited state (QD with one electron-hole pair-state 1) is observed as a function of the power of the laser excitation. The oscillation is damped due to the finite lifetime of the system and to excitation induced dephasing.

Future developments shift towards spin-photon entanglement, entanglement between distant qubits and integration of the qubits into photonic circuits like photonic crystals (for efficient and versatile light-matter coupling), waveguides for transmission of photons, realization of interconnected networks. Our main goal for the next years is to use QDs as elementary bricks for quantum information protocols. Site-controlled and charge – controlled QDs will be deterministically embedded into photonic crystal devices. This project has been funded by the French ANR and involves MBE growth, design and fabrication of nanophotonic structures. If successful, the project will open the way to an all-optical coherent manipulation of the spin in order to demonstrate entanglement between two distant spins mediated by photons all on-chip.

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Caption : schematic representation of a Lamda-system and the spin-photon interface. The spin state is prepared in level g2 ; laser pulses control the spin qubit and the polarization of the emitted photon is entangled with the corresponding spin projection. Right : schematic view of the all-on chip integrated device demonstrating entanglement between distant spins.

1] L. Monniello, C. Tonin, R. Hostein, A.Lemaitre, A. Martinez, V. Voliotis, R. Grousson, Excitation-Induced Dephasing in a Resonantly Driven InAs/GaAs Quantum Dot, Phys. Rev. Lett. 111, 026403 (2013)
[2] A. Reigue, F. Lux, L. Monniello, M. Bernard, F. Margaillan, A. Lemaître, J.Iles-Smith, J. Mork, R. Hostein, and V. Voliotis, Probing exciton-photon interaction by two-photon interferences in a resonantly driven quantum dot, Phys. Rev. Lett. 118, 233602 (2017)